1. Field of the Invention
The present invention relates to an air passage opening and closing device for opening and closing an air passage by a rotatable door. The present invention also relates to an air conditioner for vehicle use into which the air passage opening and closing device is incorporated. More particularly, the present invention relates to a device preferably used for a blowing mode switching mechanism in an air conditioner for vehicle use.
2. Description of the Related Art
The present inventors have already proposed an air conditioner for vehicle use, having a blowing mode switching mechanism, shown in FIG. 13 of Japanese Patent Application No. 2003-122281. According to this prior art, there is provided a blowing mode switching mechanism for opening and closing a defroster opening portion 20, a face opening portion 21 and a foot opening portion 22, and two rotary doors 25, 26 are arranged in this blowing mode switching mechanism.
These two rotary doors 25, 26 are respectively rotated round the rotary shafts 25b, 26b. The outer circumferential door faces 25e, 26e are arranged in portions distant from the centers of these rotary shafts 25b, 26b in the radial direction. Both end portions of these outer circumferential door faces 25e, 26e in the axial direction are connected to the rotary shafts 25b, 26b. 
Due to the above structure, the outer circumferential door faces 25e, 26e are rotated integrally with the rotary shafts 25b, 26b. The first rotary door 25, which is one of the two rotary doors 25, 26, is located on the upstream side of the air current and opens and closes an inlet passage of the front foot opening portion 22 and the rear foot opening portion not shown.
The second rotary door 26 located on the downstream side of the air current opens and closes the defroster opening portion 20 and the face opening portion 21. Further, by being linked with the foot opening portion 22, the first rotary door 25 opens and closes the communication passage opening portion 27 in the upstream portion of the second rotary door 26.
FIG. 13 is a view showing a state at the time of the face mode operation. The first rotary door 25 completely closes an inlet passage of the foot opening portion 22 and at the same time completely opens the communication passage opening portion 27. The second rotary door 26 completely closes the defroster opening portion 20 and at the same time completely opens the face opening portion 21. Due to the foregoing, a current of air flowing in the case 11 passes through the communication passage opening portion 27 and the face opening portion 21 as shown by arrow Y and blows out to the upper half of the passenger in the vehicle compartment.
In this connection, positions of the centers of gravity of the first 25 and the second rotary door 26 shown in FIG. 13 at the rotary positions are respectively G1 and G2. The door weights W1 and W2 act downward at these positions of the centers of gravity G1 and G2. As a result, the axial moments M1 and M2 act on the first 25 and the second rotary door 26 round the rotary shafts 25b, 26b. 
Therefore, the following problems may be encountered. In the case where the first rotary door 25 and the second rotary door 26 are rotated in the directions D and E, which are reverse to the directions of the axial moments M1 and M2, an intensity of the force to operate the door is increased due to by the axial moments M1 and M2.
In order to solve the above problems, the present inventors made investigations as follows. As shown in FIG. 13, the coil-shaped tension springs 51, 52, which give the spring loads P1 and P2 in the directions reverse to the axial moments M1 and M2, are set at the driving levers 38, 39 connected to the rotary shafts 25b, 26b of the doors 25, 26.
The present inventors also made investigations as follows. As shown in FIG. 14, when the fork-shaped coil springs 53, 54 are respectively set corresponding to the levers 38, 39, the spring loads P1 and P2 in the direction reverse to the axial moments M1 and M2 are given to the levers 38, 39 of the doors 25, 26.
However, in the examples of the above investigations, the exclusively used springs are added corresponding to the doors 25, 26. Therefore, the number of parts is increased. Accordingly, the assembling property is deteriorated.
In this connection, in FIGS. 13 and 14, the door means for opening and closing the air passage is composed of the rotary doors 25, 26, the outer circumferential door faces 25e, 26e of which rotate in the direction perpendicular to the air flow, and these outer circumferential door faces 25e, 26e are distant from the centers of the rotary shafts 25b, 26b by a predetermined distance. However, in the case where the door means for opening and closing the air passage is composed of a cantilever plate door, the rotary shaft of which is arranged at the end portion of the plate door body, the following problem may be encountered. As the plate door body is rotated while resisting the wind pressure generated by the air flow, the intensity of the door operating force is increased by the wind pressure.